1. Field of the Invention
The present invention relates to a fuel cell system and a method of controlling the same and, more particularly, to a fuel cell system and a method of controlling the same providing improved response time.
2. Description of the Related Art
In related art, a fuel cell system includes a reformer for reforming feedstock such as methanol to produce hydrogen rich reformed gas, a combustor for supplying heat to the reformer, and a fuel cell for reacting the reformed gas and oxygen gas contained in air supplied from an air supply unit to produce electric power.
In such a fuel cell system, the fuel cell is usually supplied with excessive amounts of reformed gas and air and all of the reformed gas and air are not consumed. Non-reacted reformed gas and air are exhausted from the fuel cell and returned to the combustor to be combusted. The combustor combusts them to generate heat, which is supplied to the reformer. In Japanese Patent application Laid-Open Publications H8-273685 and H10-106607, it has been proposed to control the flow rate of air to be supplied to the combustor with a view to controlling the output temperature of the combustor to a target temperature.
In the combustor of the above fuel cell system, it has been proposed to utilize reformed gas containing hydrogen rich gas produced in the fuel cell system, as one of fuels. An ingredient concentration, namely, hydrogen and CO concentration of the reformed gas usually varies in dependence on the operating condition of the fuel cell system. As the ingredient concentration of the reformed gas varies, the heating value of the combustor changes, while disturbing the output temperature of the combustor with a resultant unbalance caused in the whole operation of the fuel cell system.
The variation in the ingredient concentration of the reformed gas causes a serious disturbance when the output temperature of the combustor is controlled at a target temperature. Since the serious disturbance causes an unstable operation in a temperature control system of the combustor, it is difficult to set a control gain having a relatively large value. If, however, the control gain does not have the larger value, the response time of the fuel cell system is extremely deteriorated.
In the above fuel cell system and control method thereof, however, careful consideration has not been given to the fact that the temperature control of the combustor is adversely affected by the disturbance caused by the variation in the ingredient concentration of the reformed gas. As a result, the control gain should be restricted in a small range and, accordingly, the response time of the temperature control is sacrificed.
Besides, it is difficult to measure the ingredient concentration of the reformed gas on a real time basis. To cover this difficulty, an ingredient concentration presumption (estimation) unit may be provided that generate an ingredient concentration presumption output. This output may be utilized for compensating a control parameter. However, the ingredient concentration presumption unit has drawback in that there exists a deviation between a presumed (estimated) output and a current ingredient concentration of the reformed gas. This deviation causes a serious non-measurable disturbance in the fuel cell system.
In order to remove the non-measurable disturbance, an integrating control unit may be provided in the fuel cell system. However, even in such case, the response time of the temperature control in the combustor are seriously deteriorated. This is due to the fact that, when there exists the disturbance owing to the deviation in the output of the ingredient concentration presumption unit, an integral term to be calculated in the integrating control unit should have a large value in order to overcome malfunctions caused by the disturbance. Consequently, the control deviation value tends to decrease, thereby deteriorating the response time of the temperature control unit of the combustor wherein the output temperature should quickly respond to the target temperature especially when the target temperature is varied.
The present invention has been made in view of the above-described inadequacies of the fuel cell system and the method of controlling the same and has an object to provide a fuel cell system and a method of controlling the same, which overcome and eliminate the inadequacies discussed above. Namely, it is an object of the present invention to provide a fuel cell system and a method of controlling the same, wherein the fuel cell system includes a reformed gas ingredient concentration presumption unit adapted to control the output temperature of a combustor so as to restrain the influence due to the disturbance caused by the variation in the ingredient concentration of the reformed gas and the reformed gas ingredient concentration presumption unit can not be adversely affected by disturbances caused by a deviation in the ingredient concentration presumption unit when the ingredient concentration of the reformed gas changes, for thereby providing highly improved response time.
One aspect of the present invention is a fuel cell system provided with: a reformer reforming fuel to produce reformed gas; a combustor supplying heat to the reformer; an air supply unit supplying air; a fuel cell producing electric power by reacting the reformed gas and an oxygen gas contained in the air supplied by the air supply unit; an exhaust system returning exhaust reformed gas and exhaust air from the fuel cell to the combustor; an exhaust reformed gas ingredient concentration presumption unit calculating ingredient concentration of the exhaust reformed gas by using given parameters and producing a presumed output representing the ingredient concentration of the exhaust reformed gas; a combustor""s output temperature detector detecting an output temperature of the combustor and producing a combustor""s output temperature detection output; a combustor temperature controller controlling the output temperature of the combustor in response to the presumed output of the ingredient concentration of the exhaust reformed gas and the combustor""s output temperature detection output at a predetermined target temperature; a parameter correction discriminating section discriminating whether correction is required in the given parameters of the exhaust reformed gas ingredient concentration presumption unit and producing a parameter correction discriminating output; and a parameter correction data calculating section producing a parameter correction data in response to the parameter correction discriminating output to correct the given parameters of the exhaust reformed gas ingredient concentration presumption unit.
In other words, a fuel cell system of the present invention includes: means for calculating ingredient concentration of the exhaust reformed gas by using given parameters and producing a presumed output representing the ingredient concentration of the exhaust reformed gas; means for detecting an output temperature of the combustor and producing a combustor""s output temperature detection output; means for controlling the output temperature of the combustor in response to the presumed output and the combustor""s output temperature detection output at a predetermined target temperature; means for discriminating whether correction is required in the given parameters and producing a parameter correction discriminating output; and means for producing a parameter correction data in response to the parameter correction discriminating output to correct the given parameters.
Besides, a method of controlling a fuel cell system of the present invention calculates ingredient concentration of the exhaust reformed gas by using given parameters and producing a presumed output representing the ingredient concentration of the exhaust reformed gas; detects an output temperature of the combustor and producing a combustor""s output temperature detection output; controls the output temperature of the combustor in response to the presumed output and the combustor""s output temperature detection output at a predetermined target temperature; discriminates whether correction is required in the given parameters and producing a parameter correction discriminating output; and produces a parameter correction data in response to the parameter correction discriminating output to correct the given parameters.
Other and further features, advantages, and benefits of the invention will become more apparent from the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of the invention, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention in general terms. Incidentally, like numerals refer to like parts throughout the disclosure.